Environmentally benign production of graphene oxide

ABSTRACT

A method of producing isolated graphene oxide sheets directly from a graphitic material, comprising: a) mixing multiple particles of a graphitic material, an optional oxidizing liquid, and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; b) operating the energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from the graphitic material and transferring the graphene sheets to surfaces of the solid carrier material particles to produce graphene-coated solid carrier particles inside the impacting chamber; and c) sequentially or concurrently oxidizing and separating the graphene sheets from the solid carrier material particle surfaces to produce isolated graphene oxide sheets. The process is fast (1-4 hours as opposed to 5-120 hours of conventional processes), has low or no water usage, environmentally benign, cost effective, and highly scalable.

FIELD OF THE INVENTION

The present invention relates to the art of graphene materials and, inparticular, to a method of producing isolated graphene oxide sheets inan environmentally benign manner.

BACKGROUND

A single-layer graphene sheet is composed of carbon atoms occupying atwo-dimensional hexagonal lattice. Multi-layer graphene is a plateletcomposed of more than one graphene plane. Individual single-layergraphene sheets and multi-layer graphene platelets are hereincollectively called nano graphene platelets (NGPs) or graphenematerials. NGPs include pristine graphene (essentially 99% of carbonatoms), slightly oxidized graphene (<5% by weight of oxygen), grapheneoxide (≥5% by weight of oxygen), slightly fluorinated graphene (<5% byweight of fluorine), graphene fluoride ((≥5% by weight of fluorine),other halogenated graphene, and chemically functionalized graphene.

Our research group was among the first to research graphene [B. Z. Jangand W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent applicationSer. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No.7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGPnanocomposites were reviewed by us [Bor Z. Jang and A Zhamu, “Processingof Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J.Materials Sci. 43 (2008) 5092-5101].

The primary approach to production of graphene oxide is based on theintercalation and oxidation of graphite, as illustrated in FIG. 1. Thisapproach entails treating natural graphite powder with an intercalantand an oxidant (e.g., concentrated sulfuric acid and nitric acid,respectively) to obtain a graphite intercalation compound (GIC) or,actually, graphite oxide (GO). [William S. Hummers, Jr., et al.,Preparation of Graphitic Oxide, Journal of the American ChemicalSociety, 1958, p. 1339.] Prior to intercalation or oxidation, graphitehas an inter-graphene plane spacing of approximately 0.335 nm (L_(d)=1/2d₀₀₂=0.335 nm). With an intercalation and oxidation treatment, theinter-graphene spacing is increased to a value typically greater than0.6 nm. This is the first expansion stage experienced by the graphitematerial during this chemical route. The obtained GIC or GO is thensubjected to further expansion (often referred to as exfoliation) usingeither a thermal shock exposure or a solution-based,ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to ahigh temperature (typically 800-1,050° C.) for a short period of time(typically 15 to 60 seconds) to exfoliate or expand the GIC or GO forthe formation of exfoliated or further expanded graphite, which istypically in the form of a “graphite worm” composed of graphite flakesthat are still interconnected with one another. This thermal shockprocedure can produce some separated graphite flakes or graphene sheets,but normally the majority of graphite flakes remain interconnected.Typically, the exfoliated graphite or graphite worm is then subjected toa flake separation treatment using air milling, mechanical shearing, orultrasonication in water. Hence, approach 1 basically entails threedistinct procedures: first expansion (oxidation or intercalation),further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GOpowder is dispersed in water or aqueous alcohol solution, which issubjected to ultrasonication. It is important to note that in theseprocesses, ultrasonification is used after intercalation and oxidationof graphite (i.e., after first expansion) and typically after thermalshock exposure of the resulting GIC or GO (after second expansion).Alternatively, the GO powder dispersed in water is subjected to an ionexchange or lengthy purification procedure in such a manner that therepulsive forces between ions residing in the inter-planar spacesovercome the inter-graphene van der Waals forces, resulting in graphenelayer separations.

There are several major problems associated with this conventionalchemical production process:

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or sodium chlorate.    -   (2) The chemical treatment process requires a long intercalation        and oxidation time, typically 5 hours to five days.    -   (3) Strong acids consume a significant amount of graphite during        this long intercalation or oxidation process by “eating their        way into the graphite” (converting graphite into carbon dioxide,        which is lost in the process). It is not unusual to lose 20-50%        by weight of the graphite material immersed in strong acids and        oxidizers. This is also a main reason why an excessively large        amount of concentrated acid and oxidizer is required.    -   (4) The thermal exfoliation requires a high temperature        (typically 800-1,200° C.) and, hence, is a highly        energy-intensive process.    -   (5) Both heat- and solution-induced exfoliation approaches        require a very tedious washing and purification step. For        instance, typically 2.5 kg of water is used to wash and recover        1 gram of GIC, producing huge quantities of waste water that        need to be properly treated.    -   (6) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph. During the        high-temperature exfoliation, the residual intercalant species        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.

Hence, an urgent need exists to have a graphene oxide production processthat requires a reduced amount of undesirable chemical (or eliminationof these chemicals all together), shortened process time, less energyconsumption, lower degree of graphene oxidation, reduced or eliminatedeffluents of undesirable chemical species into the drainage (e.g.,sulfuric acid) or into the air (e.g., SO₂ and NO₂).

Our research has yielded a process for substantially chemical-freeproduction of isolated graphene oxide. In addition, the process is ofenhanced utility in that it is cost effective, and provides grapheneoxide materials with significantly reduced environmental impact.

SUMMARY OF THE INVENTION

The present invention provides a strikingly simple, fast, scalable,environmentally benign, and cost-effective process that meets theaforementioned needs. This method of producing single-layer or few layergraphene oxide directly from a graphitic or carbonaceous material (agraphene source material) comprises subjecting a mixture of graphiticmaterial, an optional oxidizing liquid, particles of a solid carriermaterial, and, optionally, impacting balls to mechanical agitation via aball mill or similar energy impacting device for a length of timesufficient for peeling off graphene layers (planes of hexagonallyarranged carbon atoms) from the source graphite material, and coatingthese peeled-off graphene layers onto surfaces of the solid carriermaterial particles. With the presence of impacting balls, graphenesheets can be peeled off from the source graphite particles andtentatively deposited onto the surfaces of impacting balls. When thesegraphene sheet-coated impacting balls subsequently impinge upon solidcarrier particles, the graphene sheets are transferred to surfaces ofcarrier particles. These steps are followed by sequentially orconcurrently oxidizing and separating graphene sheets from solid carriermaterial particle surfaces to produce the isolated graphene oxidesheets.

Thus, in some embodiments, the invention provides a method of producingisolated graphene oxide sheets directly from a graphitic material, themethod comprising: a) mixing multiple particles of a graphitic material,an optional oxidizing liquid, and multiple particles of a solid carriermaterial to form a mixture in an impacting chamber of an energyimpacting apparatus; b) operating the energy impacting apparatus with afrequency and an intensity for a length of time sufficient for peelingoff graphene sheets from the graphitic material and transferring thegraphene sheets to surfaces of the solid carrier material particles toproduce graphene-coated solid carrier particles inside the impactingchamber; and c) sequentially or concurrently oxidizing and separatingthe graphene sheets from the surfaces of the solid carrier materialparticles to produce the isolated graphene oxide sheets.

In some embodiments, a plurality of impacting balls or media are addedto the impacting chamber of the energy impacting apparatus.

In some embodiments, step (a) comprises adding an oxidizing liquid inthe mixture so that the oxidizing liquid acts to partially oxidize thegraphene sheets during step (b). The oxidizing liquid may be selectedfrom a liquid containing an oxidizer selected from H₂O₂, nitric acid,potassium permanganate, sodium permanganate, transition metalpermanganate, sodium chlorate, potassium chlorate, or a combinationthereof.

In some embodiments, step (c) comprises exposing the graphene sheets toan oxidizing medium, before, during or after the graphene sheets areseparated from the solid carrier material particle surfaces, wherein theoxidizing medium is selected from an oxidizing gas or vapor, anoxidizing plasma, or an oxidizing liquid. Again, the oxidizing liquidmay be selected from a liquid containing an oxidizer selected from H₂O₂,nitric acid, potassium permanganate, sodium permanganate, transitionmetal permanganate, sodium chlorate, potassium chlorate, or acombination thereof.

Preferably, the starting material (graphitic or carbonaceous material asa graphene source material) has never been previously intercalated orchemically oxidized. This starting material is not a graphiteintercalation compound (GIC) or graphite oxide (GO). Preferably, thesource graphitic material may be selected from natural graphite,synthetic graphite, highly oriented pyrolytic graphite, mesocarbonmicrobead, graphite fiber, graphitic nanofiber, graphite oxide, graphitefluoride, chemically modified graphite, exfoliated graphite, veingraphite, or a combination thereof.

In some embodiments, the impacting chamber of the energy impactingapparatus further contains a protective fluid; e.g. inert gas,non-reactive liquid, water, etc.

This is essentially a two-step process, significantly reducing processcosts. In less than 1-4 hours of process time, graphene sheets arepeeled off from graphite particles, followed by a fast, efficientremoval of the carrier material. This process is stunningly short andsimple.

A preferred embodiment of the present invention is a method of directlymixing a graphitic material and a carrier material into an energyimpacting device, such as a ball mill, and submitting the mixture to asufficiently long treatment time to peel off graphene layers from thesource graphitic material and transfer these graphene layers immediatelyto the carrier material surfaces. These graphene layers are typicallysingle-layer or few-layer graphene sheets (typically <5 layers; mostlysingle-layer graphene).

Following this step, the carrier layer may be removed by dissolution,burning, sublimation, melting or other method. The end result isisolated graphene sheets, which can then be oxidized by, for instance,immersing the isolated graphene sheets in an oxidizing liquid (e.g. 30%H₂O₂ in water).

In certain embodiments, the graphene sheets supported on carrierparticle surfaces may be exposed to the oxidizing medium before orduring the separation procedure. A very small amount of an oxidizingagent is required to accomplish the oxidation of isolated graphenesheets or graphene sheets supported on a solid carrier particle surface.

This is a surprisingly simple method. In contrast, prior researchers andmanufacturers have focused on more complex, time intensive and costlymethods to create graphene oxide in industrial quantities. The presentinvention has several unexpected attributes or advantages:

-   -   (1) Unlike the chemical intercalation and oxidation (which        requires expansion of inter-graphene spaces, further expansion        or exfoliation of graphene planes, and full separation of        exfoliated graphene sheets), the instant method directly removes        graphene sheets from a source graphitic material and transfers        these graphene sheets to surfaces of carrier material particles.        No undesirable chemicals (e.g. sulfuric acid and nitric acid)        are used in the production of isolated graphene sheets.    -   (2) Unlike conventional oxidation and intercalation methods        (e.g. well-known Hummer's method) that always produce heavily        oxidized graphene sheets (i.e. heavily damaged graphene oxide,        having an oxygen content up to 47% by weight) in a        non-controllable manner, initially pristine graphene sheets are        produced by using the invented approach. These pristine graphene        sheets can be subsequently oxidized in a controlled manner to        yield GO sheets of controllable and desirable oxygen contents        from 5% to approximately 50% by weight. The GO sheets produced        by conventional methods typically cannot recover their structure        and properties even after thermal or chemical reduction of GO        sheets. In contrast, the sheets being initially free of        oxidation damage allow the creation of graphene containing        products with higher electrical and thermal conductivity.    -   (3) The amount of sulfuric acid/oxidizer used is typically only        1-10% of what would be required for GO production using        conventional methods. Furthermore, more environmentally benign        oxidizers (e.g. H₂O₂) can be readily and easily used, instead of        undesirable chemicals such as strong sulfuric acids and nitric        acid or potassium permanganate.    -   (4) Consequently, the required amounts of washing water are        dramatically reduced. Furthermore, the amounts of NO_(x) and        SO_(x) resulting from residual sulfuric acid and nitric acid of        conventional methods are dramatically reduced or even totally        eliminated.

The carrier material can be an organic, inorganic, metal, glass, metaloxide, metal carbide, metal nitride, metal boride, or ceramic material.Carrier materials can be in the form of pellets, filament, fibers,powder, reactor spheres, or other forms. Some examples of organiccarrier materials include wax, soluble polymers, nut husks, and woodchips.

The energy impacting apparatus may be selected from a ball mill,vibratory ball mill, planetary ball mill, high energy mill, basket mill,agitator ball mill, continuous ball mill, stirred ball mill, pressurizedball mill, vacuum ball mill, freezer (SPEX) mill, vibratory sieve,ultrasonic homogenizer mill, resonant acoustic mixer, or shaker table.

The presently invented process is capable of producing single-layergraphene sheets. In many examples, the graphene material producedcontains at least 80% single-layer graphene sheets. The grapheneproduced can contain pristine graphene, oxidized graphene with less than5% oxygen content by weight, graphene fluoride with less than 5%fluorine by weight, graphene with a carbon content no less than 95% byweight, or functionalized graphene.

Another surprising and highly advantageous feature of the presentlyinvented process is the notion that graphene sheet production andoxidation can be accomplished concurrently in the same impacting chamberprovided an oxidizing agent is enclosed in the impacting chamber. Theimpact-induced kinetic energy experienced by the carrier particles areof sufficient energy and intensity to chemically activate the edges andsurfaces of graphene sheets coated on carrier particle surfaces; e.g.creating highly active sites or free radicals). Desiredoxygen-containing groups (e.g. —COOH, —OH, etc.) can be imparted tographene edges and/or surfaces, provided that selected chemical species(oxidizing agents) containing desired chemical function groups.) aredispersed in the impacting chamber. Chemical oxidation reactions canoccur in situ as soon as the reactive sites or active radicals areformed.

The present invented method may further contain a step offunctionalizing the produced graphene oxide sheets.

In some embodiments, functionalizing agents contain a chemicalfunctional group selected from alkyl or aryl silane, alkyl or aralkylgroup, hydroxyl group, carboxyl group, amine group, sulfonate group(—SO₃H), aldehydic group, quinoidal, fluorocarbon, or a combinationthereof.

Alternatively, the functionalizing agent contains an azide compoundselected from the group consisting of 2-azidoethanol,3-azidopropan-1-amine, 4-(2-azidoethoxy)-4-oxobutanoic acid,2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R−)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.

In certain embodiments, the functionalizing agent contains an oxygenatedgroup selected from the group consisting of hydroxyl, peroxide, ether,keto, and aldehyde. In certain embodiments, the functionalizing agentcontains a functional group selected from the group consisting of SO₃H,COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′,SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂and Mg—X; wherein y is an integer equal to or less than 3, R′ ishydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, orpoly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, and combinations thereof.

The functionalizing agent may contain a functional group is selectedfrom the group consisting of amidoamines, polyamides, aliphatic amines,modified aliphatic amines, cycloaliphatic amines, aromatic amines,anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine(TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine,polyamine epoxy adduct, phenolic hardener, non-brominated curing agent,non-amine curatives, and combinations thereof.

In some embodiments, the functionalizing agent contains a functionalgroup selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY,N′Y or C′Y, and Y is a functional group of a protein, a peptide, anamino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, anantigen, or an enzyme substrate, enzyme inhibitor or the transitionstate analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂,R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y),R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H,(—C₂H₂O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, and w is an integer greater thanone and less than 200.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart showing the most commonly used prior art process ofproducing highly oxidized NGPs that entails tedious chemicaloxidation/intercalation, rinsing, and high-temperature exfoliationprocedures.

FIG. 2 A flow chart showing the presently invented two-step process forproducing isolated graphitic materials

FIG. 3 A flow chart showing the presently invented process for producingisolated graphitic materials via a continuous ball mill.

FIG. 4(A) Transmission electron micrograph of graphene sheets producedby conventional Hummer's route (much smaller graphene sheets, butcomparable thickness).

FIG. 4(B) Transmission electron micrograph of graphene sheets producedby the presently invented impact energy method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassycarbon), a highly organized crystal (graphite), or a whole range ofintermediate structures that are characterized in that variousproportions and sizes of graphite crystallites and defects are dispersedin an amorphous matrix. Typically, a graphite crystallite is composed ofa number of graphene sheets or basal planes that are bonded togetherthrough van der Waals forces in the c-axis direction, the directionperpendicular to the basal plane. These graphite crystallites aretypically micron- or nanometer-sized. The graphite crystallites aredispersed in or connected by crystal defects or an amorphous phase in agraphite particle, which can be a graphite flake, carbon/graphite fibersegment, carbon/graphite whisker, or carbon/graphite nanofiber.

One preferred specific embodiment of the present invention is a methodof producing oxidized versions of graphene sheets or nanographeneplatelet (NGP) material. The NGP is essentially composed of a sheet ofgraphene plane (hexagonal lattice of carbon atoms) or multiple sheets ofgraphene plane stacked and bonded together (typically, on an average,less than five sheets per multi-layer platelet). Each graphene plane,also referred to as a graphene sheet or basal plane comprises atwo-dimensional hexagonal structure of carbon atoms. Each platelet has alength and a width parallel to the graphite plane and a thicknessorthogonal to the graphite plane. By definition, the thickness of an NGPis 100 nanometers (nm) or smaller, with a single-sheet NGP being as thinas 0.34 nm. However, the NGPs produced with the instant methods aremostly single-layer graphene with some few-layer graphene sheets (2-10layers, but mostly <5 layers). The length and width of a NGP aretypically between 200 nm and 20 μm, but could be longer or shorter,depending upon the sizes of source graphite material particles.

The present invention provides a strikingly simple, fast, scalable,environmentally benign, and cost-effective process that avoidsessentially all of the drawbacks associated with prior art processes. Asschematically illustrated in FIG. 2, one preferred embodiment of thismethod entails placing source graphitic material particles, an optionaloxidizing agent, and carrier material particles (plus optional impactingballs, if so desired) in an impacting chamber. After loading, theresulting mixture is immediately exposed to impacting energy, which isaccomplished by rotating the chamber to enable the impacting of thecarrier particles (and optional impacting balls) against graphiteparticles. These repeated impacting events (occurring in highfrequencies and high intensity) act to peel off graphene sheets from thesurfaces of graphitic material particles and directly transfer thesegraphene sheets to the surfaces of carrier particles. This is a “directtransfer” process. If an oxidizing agent is present in the impactingchamber, oxidation of the produced graphene sheets also occurssubstantially concurrently.

In certain embodiments, in the impacting chambers containing impactingballs (e.g. stainless steel or zirconia beads), graphene sheets arepeeled off by the impacting balls and tentatively transferred to thesurfaces of impacting balls first. When the graphene-coated impactingballs impinge upon the carrier material particles, the graphene sheetsare transferred to surfaces of the carrier material particles. This isan “indirect transfer” process.

In less than 1-4 hours, most of the constituent graphene sheets ofsource graphite particles are peeled off, forming mostly single-layergraphene and few-layer graphene (mostly less than 5 layers). Followingthe direct or indirect transfer process (coating of graphene sheets oncarrier material particles), the graphene sheets can be separated fromthe carrier material particles using a broad array of methods. Forinstance, the carrier material (e.g. plastic or organic material) isignited, burning away the carrier material and leaving behind isolatednanographene platelets. The carrier material (e.g. wax, polymer, orsalt) may be dissolved in a benign solvent (e.g. water, if the carrieris a water soluble material). There are many water soluble polymers(e.g. polyacrylamide and polyvinyl alcohol) and many water soluble salts(e.g. NaCl) that can be used for this purpose. If necessary, organicsolvents can be used to dissolve most of the thermoplastic materials.The graphene sheets coated on metal, glass, or ceramic particles (metaloxide, metal carbide, etc.) can be easily detached from the particles byimmersing the coated particles in a water bath and subjected the slurryto mild ultrasonication, for instance. Oxidation of graphene sheets canbe allowed to occur before or during the detaching procedure. Thisgraphene oxide production process is stunningly short and simple, andhighly scalable.

In contrast, as shown in FIG. 1, the prior art chemical processestypically involve immersing graphite powder in a mixture of concentratedsulfuric acid, nitric acid, and an oxidizer, such as potassiumpermanganate or sodium perchlorate, forming a reacting mass thatrequires typically 5-120 hours to complete the chemicalintercalation/oxidation reaction. Once the reaction is completed, theslurry is subjected to repeated steps of rinsing and washing with waterand then subjected to drying treatments to remove water. The driedpowder, referred to as graphite intercalation compound (GIC) or graphiteoxide (GO), is then subjected to a thermal shock treatment. This can beaccomplished by placing GIC in a furnace pre-set at a temperature oftypically 800-1100° C. (more typically 950-1050° C.). The resultingproducts are typically highly oxidized graphene (i.e. graphene oxidewith a high oxygen content), which must be chemically or thermal reducedto obtain reduced graphene oxide (RGO). RGO is found to contain a highdefect population and, hence, is not as conducting as pristine graphene.We have observed that that the pristine graphene paper (prepared byvacuum-assisted filtration of pristine graphene sheets) exhibitelectrical conductivity values in the range from 1,500-4,500 S/cm. Incontrast, the RGO paper prepared by the same paper-making proceduretypically exhibits electrical conductivity values in the range from100-1,000 S/cm.

It is again critically important to recognize that the impacting processnot only avoids significant chemical usage, but also produces a higherquality final product—pristine graphene as opposes to thermally reducedgraphene oxide, as produced by the prior art process. Pristine grapheneenables the creation of GO materials in a controlled manner.

Although the mechanisms remain incompletely understood, thisrevolutionary process of the present invention appears to essentiallyeliminate the required functions of graphene plane expansion,intercalant penetration, exfoliation, and separation of graphene sheetsand replace it with an entirely mechanical exfoliation process. Thewhole process can take less than 1-4 hours, and can be done with noadded undesirable chemicals. This is absolutely stunning, a shockingsurprise to even those top scientists and engineers or those ofextraordinary ability in the art.

Another surprising result of the present study is the observation that awide variety of carbonaceous and graphitic materials can be directlyprocessed without any particle size reduction or pre-treatment. Thismaterial may be selected from natural graphite, synthetic graphite,highly oriented pyrolytic graphite, mesocarbon microbead, graphitefiber, graphitic nanofiber, graphite oxide, graphite fluoride,chemically modified graphite, exfoliated graphite, or a combinationthereof. By contrast, graphitic material for used for the prior artchemical formation and reduction of graphene oxide requires sizereduction to 75 um or less average particle size. This process requiressize reduction equipment (for example hammer mills or screening mills),energy input, and dust mitigation. By contrast, the energy impactingdevice method can accept almost any size of graphitic material. Startingmaterial of mm or cm in size or larger has been successfully processedto create graphene. The only size limitation is the chamber capacity ofthe energy impacting device.

The presently invented process is capable of producing single-layergraphene oxide sheets. In many examples, the graphene material producedcontains at least 80% (can be higher than 90%) single-layer grapheneoxide sheets.

The presently invented process does not involve the production of GICand, hence, does not require the exfoliation of GIC at a highexfoliation temperature (e.g. 800-1,100° C.). This is another majoradvantage from environmental protection perspective. The prior artprocesses require the preparation of dried GICs containing sulfuric acidand nitric acid intentionally implemented in the inter-graphene spacesand, hence, necessarily involve the decomposition of H₂SO₄ and HNO₃ toproduce volatile gases (e.g. NO_(x) and SO_(x)) that are highlyregulated environmental hazards. The presently invented processcompletely obviates the need to decompose H₂SO₄ and HNO₃ and, hence, isenvironmentally benign. No undesirable gases are released into theatmosphere during the combined graphite expansion/exfoliation/separationprocess of the present invention.

One preferred embodiment of the present invention is the inclusion ofimpacting balls or media to the impacting chamber, as illustrated inFIG. 2. The impact media may contain balls of metal, glass, ceramic, ororganic materials. The size of the impacting media may be in the rangefrom 1 mm to 20 mm, or it may be larger or smaller. The shape of theimpacting media may be spherical, needle like, cylindrical, conical,pyramidal, rectilinear, or other shapes. A combination of shapes andsizes may be selected. The size distribution may be unimodal Gaussian,bimodal or tri-modal.

Another preferred embodiment of this method is removing the carriermaterial through burning, leaving behind isolated nanographeneplatelets. Unlike the high energy input required for the prior artprocess to reach 1050° C. to thermally reduce graphene oxide, burn outof organic materials is an exothermic process. Once initiated in acontinuous oven or furnace, the energy required to maintain burn outtemperature is modest and does not impact the cost effectiveness of theprocess.

Another preferred embodiment of this method is removing the carriermaterial through a mixture of burn out (oxygen atmosphere) and reduction(inert or reducing atmosphere). In this embodiment, the carrier materialis ignited at room temperature or in an oven. The oven temperature ispreferably 30° C. to 1000° C., more preferably 200° C.-800° C., and morepreferably 350° C.-450° C. The burn out process removes 90 to 99.9% ofthe carrier material. In this embodiment, a second thermal process iscarried out at greater than 600° C. in an inert or reducing atmosphereto remove remaining carrier material.

Another preferred embodiment of this method is removing the carriermaterial through melting, leaving behind isolated nanographeneplatelets. This process has allows for re-use of the carrier material,reducing process cost and environmental impact. Unlike the hightemperatures required for the prior art process of thermal reduction ofgraphene oxide to produce graphene, the temperatures required for amelting process are comparatively low: for example 46° C. to 200° C. forwax, 231.9° C. for tin, 166° C. for poly lactic acid polymer. The energyinput required to remove the carrier from graphene is modest and doesnot impact the cost effectiveness of the process.

Another preferred embodiment of this method is removing the carriermaterial through dissolution. For example,acrylonitrile-butadiene-styrene (ABS) carrier material can be dissolvedin acetone. After washing over a filter to remove residual ABS, thefiltrate can be vacuum dried, spray dried, or air dried, resulting inisolated graphene material. The solvents used in this process can berecovered via solvent recovery processes including vacuum distillation,this is a cost effective process.

Another preferred embodiment of this method is removing the carriermaterial through sublimation. The carrier material may be selected fromice or iodine crystals. If ice is used as the carrier material, acommercially available freeze dryer can be used to remove the carriervia sublimation. A major benefit of the sublimation approach is theavoidance of the exposure of NGP to surface tension, which can causemorphology changes or agglomeration. Another benefit of sublimationprocess is the ability to choose a carrier material that will leave nochemical residue on the graphene material.

One significant advantage of the present invention as compared to priorart is flexibility of selecting carrier materials. There are manyopportunities to use pre-consumer or post-consumer waste material as thecarrier, diverting this material from disposal by landfill orincineration. Nut shells, rice husks, shredded tires, and groundco-mingled recycled plastic are all possible cost effective carriermaterials for the production of graphene. An additional advantage ofthis flexibility is the carrier material can be chosen to maximizeburnout or to maximize solubility in low cost, environmentally friendly,and lower health hazard hazardous solvents. For example, a carriermaterial readily soluble in acetone may be chosen over a carriermaterial requiring methyl ethyl ketone for removal, and will facilitatethe scale up of a cost effective manufacturing process.

An important advantage of the present invention is the opportunity touse multiple carrier removal methods. For example, melting at 200° C.may be used to remove 99% of a wax carrier material, followed by burnoutat 1000° C. As another example, solvent removal may be used to removethe majority of a polymer carrier material, followed by burnout at 1200°C. The combining of carrier removal processes allows for cost effectivere-use of carrier materials, while also achieving high levels of purityfor the final graphene produced.

In a desired embodiment, the presently invented method is carried out inan automated and/or continuous manner. For instance, as illustrated inFIG. 3, the mixture of graphite particles and solid carrier particles(plus optional impacting balls) is delivered by a conveyer belt 12 andfed into a continuous ball mill 14. After ball milling to formgraphene-coated solid carrier particles, the product mixture (possiblyalso containing some residual graphite particles and optional impactingballs) is discharged from the ball mill apparatus 14 into a screeningdevice (e.g. a rotary drum 16) to separate graphene-coated solid carrierparticles from residual graphite particles (if any) and impacting balls(if any). This separation operation may be assisted by a magneticseparator 18 if the impacting balls are ferromagnetic (e.g. balls of Fe,Co, Ni, or Mn-based metal). The graphene-coated carrier particles may bedelivered into a combustion chamber 20, if the solid carrier can beburned off (e.g. plastic beads, rubber particles, and wax particles,etc.). Alternatively, these particles can be discharged into adissolving chamber for dissolving the carrier particles (e.g. plasticbeads). The product mass can be further screened in another (optional)screening device 22, a powder classifier or cyclone 24, and/or anelectrostatic separator 26. These procedures can be fully automated.

FIG. 4(A) shows a transmission electron micrograph of graphene sheetsproduced by conventional Hummer's route (much smaller graphene sheets,but comparable thickness). FIG. 4(B) shows a transmission electronmicrograph of graphene sheets produced by the presently invented impactenergy method, which are larger in length and width.

Preferred Mode of Chemical Functionalization

Graphene sheets transferred to carrier solid particle surfaces, beforeor after separation, have a significant proportion of surfaces thatcorrespond to the edge planes of graphite crystals. The carbon atoms atthe edge planes are reactive and must contain some heteroatom or groupto satisfy carbon valency. There are many types of functional groups(e.g. hydroxyl and carboxylic) that are naturally present at the edge orsurface of graphene nanoplatelets produced through transfer to a solidcarrier particle. The impact-induced kinetic energy experienced by thecarrier particles are of sufficient energy and intensity to chemicallyactivate the edges and even surfaces of graphene sheets coated oncarrier particle surfaces (e.g. creating highly active sites or freeradicals). These conditions enable ready oxidation of the graphenesheets (free-standing or being supported on a solid carrier surface) inthe presence of an oxidizing agent in the impacting chamber. n addition,if certain chemical species containing desired chemical function groups(e.g. —NH₂, Br—, etc.) are added in the impacting chamber (preferablyafter oxidation of graphene occurs), these functional groups can beimparted to graphene edges and/or surfaces. In other words, productionand chemical functionalization of graphene oxide sheets can beaccomplished concurrently by including appropriate chemical compounds inthe impacting chamber. In summary, a major advantage of the presentinvention over other processes is the simplicity of simultaneousproduction and modification of surface chemistry.

Graphene oxide sheets, after being recovered from the impacting chamber,may be functionalized through various chemical functionalizationtreatments. In each group of chemical species discussed below, weselected 2 or 3 chemical species for functionalization studies.

In one preferred group of chemical agents, the resulting functionalizedNGP may broadly have the following formula(e): [NGP]—R_(m), wherein m isthe number of different functional group types (typically between 1 and5), R is selected from SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl,halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—OR′—)_(y)R′_(3-y),Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is aninteger equal to or less than 3, R′ is hydrogen, alkyl, aryl,cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ isfluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, Xis halide, and Z is carboxylate or trifluoroacetate.

For NGPs to be effective reinforcement fillers in epoxy resin, thefunction group —NH₂ is of particular interest. For example, a commonlyused curing agent for epoxy resin is diethylenetriamine (DETA), whichhas three —NH₂ groups. If DETA is included in the impacting chamber, oneof the three —NH₂ groups may be bonded to the edge or surface of agraphene sheet and the remaining two un-reacted —NH₂ groups will beavailable for reacting with epoxy resin later. Such an arrangementprovides a good interfacial bonding between the NGP (graphene sheets)and the matrix resin of a composite material.

Other useful chemical functional groups or reactive molecules may beselected from the group consisting of amidoamines, polyamides, aliphaticamines, modified aliphatic amines, cycloaliphatic amines, aromaticamines, anhydrides, ketimines, diethylenetriamine (DETA),triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA),hexamethylenetetramine, polyethylene polyamine, polyamine epoxy adduct,phenolic hardener, non-brominated curing agent, non-amine curatives, andcombinations thereof. These functional groups are multi-functional, withthe capability of reacting with at least two chemical species from atleast two ends. Most importantly, they are capable of bonding to theedge or surface of graphene using one of their ends and, duringsubsequent epoxy curing stage, are able to react with epoxide or epoxyresin at one or two other ends.

The above-described [NGP]—R_(m) may be further functionalized. This canbe conducted by opening up the lid of an impacting chamber after the—R_(m) groups have been attached to graphene sheets and then adding thenew functionalizing agents to the impacting chamber and resuming theimpacting operation. The resulting graphene sheets or platelets includecompositions of the formula: [NGP]-A_(m), where A is selected from OY,NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is anappropriate functional group of a protein, a peptide, an amino acid, anenzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or anenzyme substrate, enzyme inhibitor or the transition state analog of anenzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN,R′X, R′N⁺(R)₃X⁻, R′SiR'₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′,R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O)_(w)—R′,(C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than200.

The NGPs may also be functionalized to produce compositions having theformula: [NGP]—[R′—A]_(m), where m, R′ and A are as defined above. Thecompositions of the invention also include NGPs upon which certaincyclic compounds are adsorbed. These include compositions of matter ofthe formula: [NGP]—[X—R_(a)]_(m), where a is zero or a number less than10, X is a polynuclear aromatic, polyheteronuclear aromatic ormetallopolyheteronuclear aromatic moiety and R is as defined above.Preferred cyclic compounds are planar. More preferred cyclic compoundsfor adsorption are porphyrins and phthalocyanines. The adsorbed cycliccompounds may be functionalized. Such compositions include compounds ofthe formula, [NGP]—[X-A_(a)]_(m), where m, a, X and A are as definedabove.

The functionalized NGPs of the instant invention can be prepared bysulfonation, electrophilic addition to deoxygenated platelet surfaces,or metallation. The graphitic platelets can be processed prior to beingcontacted with a functionalizing agent. Such processing may includedispersing the platelets in a solvent. In some instances the plateletsmay then be filtered and dried prior to contact. One particularly usefultype of functional group is the carboxylic acid moieties, whichnaturally exist on the surfaces of NGPs if they are prepared from theacid intercalation route discussed earlier. If carboxylic acidfunctionalization is needed, the NGPs may be subjected to chlorate,nitric acid, or ammonium persulfate oxidation.

Carboxylic acid functionalized graphitic platelets are particularlyuseful because they can serve as the starting point for preparing othertypes of functionalized NGPs. For example, alcohols or amides can beeasily linked to the acid to give stable esters or amides. If thealcohol or amine is part of a di- or poly-functional molecule, thenlinkage through the O- or NH-leaves the other functionalities as pendantgroups. These reactions can be carried out using any of the methodsdeveloped for esterifying or aminating carboxylic acids with alcohols oramines as known in the art. Examples of these methods can be found in G.W. Anderson, et al., J. Amer. Chem. Soc. 86, 1839 (1964), which ishereby incorporated by reference in its entirety. Amino groups can beintroduced directly onto graphitic platelets by treating the plateletswith nitric acid and sulfuric acid to obtain nitrated platelets, thenchemically reducing the nitrated form with a reducing agent, such assodium dithionite, to obtain amino-functionalized platelets.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention:

Example 1 Isolated Graphene and Graphene Oxide Sheets from FlakeGraphite Via Polypropylene Powder-Based Carrier

In an experiment, 1 kg of polypropylene pellets, 50 grams of flakegraphite, 50 mesh (average particle size 0.18 mm; Asbury Carbons, AsburyN.J.) and 250 grams of magnetic stainless steel pins (RaytechIndustries, Middletown Conn.) were placed in a ball mill container. Theball mill was operated at 300 rpm for 4 hours. The container lid wasremoved and stainless steel pins were removed via a magnet. The polymercarrier material was found to be coated with a dark carbon layer.Carrier material was placed over a 50 mesh sieve and a small amount ofunprocessed flake graphite was removed.

Subsequently, two separate procedures were conducted to produce isolatedgraphene oxide sheets. One procedure involved placing coated carriermaterial in a crucible in a vented furnace at 600° C. to burn off PP forobtaining isolated graphene sheets. The graphene sheets were thenimmersed in a hydrogen peroxide-water solution (30% H₂O₂ in water) for aperiod of time ranging from 0.5 to 12 hours to produce GO having 5%-35%by wt. oxygen.

The other procedure entailed immersing graphene-coated PP particles inthe same hydrogen peroxide-water solution and implementing anultrasonication tip in the solution to concurrently oxidize and separategraphene sheets from the carrier particles for 15 minutes to 2 hours.The products were graphene oxide sheets having an oxygen content from 5to 35% by weight.

Although polypropylene (PP) is herein used as an example, the carriermaterial for making isolated graphene oxide sheets is not limited to PPor any polymer (thermoplastic, thermoset, rubber, etc.). The carriermaterial can be a glass, ceramic, metal, or other organic material,provided the carrier material is hard enough to peel off graphene sheetsfrom the graphitic material (if the optional impacting balls are notpresent).

Example 2 Graphene Oxide from Expanded Graphite Via ABS Polymer

In an experiment, 100 grams of ABS pellets, as solid carrier materialparticles, were placed in a 16 oz plastic container along with 5 gramsof natural graphite. This container was placed in an acoustic mixingunit (Resodyn Acoustic mixer), and processed for 30 minutes. Afterprocessing, carrier material was found to be coated with a thin layer ofgraphene.

Graphene-coated carrier particles were then immersed in a mixture ofsulfuric acid and potassium permanganate (graphene/sulfuricacid/potassium permanganate ratio of 1.0/1.0/0.5) and subjected toultrasound energy to speed oxidation of graphene and separation ofresulting GO sheets from the ABS particles. The solution was filteredusing an appropriate filter and washed with distilled water. Subsequentto washing, filtrate was dried in a vacuum oven set at 60° C. for 2hours.

It may be noted that the conventional Hummer's method typically requiresthe use of 24 mL (44 grams) of sulfuric acid and 5 grams of potassiumpermanganate to oxidize 1 gram of natural graphite. The required ratiowas 1.0/44/5; i.e. significantly higher amounts of chemicals were used.Additionally, 15-30% of the natural graphite was “eaten away” during theGO production process using the Hummer's method.

Example 3 Functionalized Graphene from Mesocarbon Micro Beads (MCMBs)Via PLA

In one example, 100 grams of PLA pellets (carrier material), 2 grams ofMCMBs (China Steel Chemical Co., Taiwan), and 2 grams of carboxylic acid(an oxidizing agent) were placed in a vibratory ball mill, which alsocontains particles of magnetic stainless steel impactor and processedfor 1 hour to obtain graphene oxide-coated PLA particles. Subsequently,DETA was added and the material mixture was processed for an additional1 hour. The vibratory mill was then opened and the carrier material wasfound to be coated with a brown-color coating of graphene oxide. Themagnetic steel particles were removed with a magnet. The carriermaterial was rinsed with isopropyl alcohol and placed on a vacuumfilter. The vacuum filter was heated to 160° C. and vacuum was applied,resulting in removal of the PLA.

In separate experiments, the following functional group-containingspecies were introduced to the graphene oxide sheets produced: an aminoacid, sulfonate group (—SO₃H), 2-azidoethanol, polyamide (caprolactam),and aldehydic group. In general, these functional groups were found toimpart significantly improved interfacial bonding between resultinggraphene oxide sheets and epoxy, polyester, polyimide, and vinyl estermatrix materials to make stronger polymer matrix composites. Theinterfacial bonding strength was semi-quantitatively determined by usinga combination of short beam shear test and fracture surface examinationvia scanning electronic microscopy (SEM). Non-functionalized graphenesheets tend to protrude out of the fractured surface without anyresidual matrix resin being attached to graphene sheet surfaces. Incontrast, the fractured surface of composite samples containingfunctionalized graphene oxide sheets do not exhibit any bare graphenesheets; any what appears to be graphene sheets were completely embeddedin a resin matrix.

Example 4 Graphene Oxide from HOPG Via Glass Beads and SPEX Mill

In an experiment, 5 grams of glass beads were placed in a SPEX millsample holder (SPEX Sample Prep, Metuchen, N.J.) along with 0.25 gramsof HOPG derived from graphitized polyimide. This process was carried outin a 1% “dry room” to reduce the condensation of water onto thecompleted product. The SPEX mill was operated for 10 minutes. Afteroperation, the contents of the sample holder were transferred to a waterbath subjected to ultrasonication, which helps to separate graphenesheets from glass bead surfaces. The remaining material in the weightdish was a mixture of single-layer graphene (86%) and few layergraphene. The graphene sheets were then oxidized in a hydrogenperoxide-water solution (30% H₂O₂ in water) for 0.5-2.0 hours.

Example 5 Production of Few Layer Graphene Oxide Via Wax-Based Carrier

In one example, 100 grams of hard machining wax in pellet form (F-14green, Machinablewax.com, Traverse City Mich., Hardness 55, Shore “D”)was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons,Asbury N.J.) and loaded into a vibratory mill. The material wasprocessed for 4 hours, and the vibratory mill was opened. The waxpellets were found to be carbon coated. These pellets were removed fromthe mill, melted, and re-pelletized, resulting in 103 grams ofgraphene-loaded wax pellets. The wax pellets were placed again in thevibratory mill with an additional 10 grams of vein graphite, andprocessed for 4 hours. The resultant material was pelletized andprocessed with an additional 10 grams of vein graphite, creating awax/graphene composite with a graphene filler level of about 8.9%. Thewax carrier material was then dissolved in hexane and transferred intoacetone via repeated washing, then separated from acetone viafiltration, producing isolated, pristine NGP. The graphene sheets weredried in a vacuum oven at 60° C. for 24 hours, and then surface area wasmeasured via nitrogen adsorption BET. The graphene sheets were thenoxidized in sodium chlorate-water solution for 0.5 hours.

Example 6 Low Temperature Metal Particles as the Carrier Material

In one example, 100 grams of tin (45 micron, 99.9% purity, GoodfellowInc.; Coraopolis, Pa.) was mixed with 10 grams of vein graphite (40 meshsize, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. Thematerial was processed for 2 hours, and the vibratory mill was opened.The tin powder was found to be carbon coated. These pellets were removedfrom the mill and poured into a hydrogen peroxide-water solution (30%H₂O₂ in water), subjected to ultrasonication. The resulting grapheneoxide sheets were filtered using a Teflon membrane. The specific surfacearea of the resulting graphene oxide was measured via nitrogenadsorption BET. A similar procedure was conducted using zinc particlesas the solid carrier material.

Example 7 Isolated Graphene and Graphene Oxide from Natural GraphiteParticles Via Polyethylene (PE) Beads and Ceramic Impacting Balls

In an experiment, 0.5 kg of PE beads (as a solid carrier material), 50grams of natural graphite (source of graphene sheets) and 250 grams ofzirconia powder (impacting balls) were placed in containers of aplanetary ball mill. The ball mill was operated at 300 rpm for 4 hours.The container lid was removed and zirconia beads (different sizes andweights than graphene-coated PE beads) were removed through a vibratoryscreen. The polymer carrier material was found to be coated with a darkcarbon layer. Carrier material was placed over a 50 mesh sieve and asmall amount of unprocessed flake graphite was removed. Coated carriermaterial was then placed in a crucible in a vented furnace at 600° C.After cooling down, the furnace was opened to reveal a crucible full ofisolated graphene sheet powder (>95% single-layer graphene). Theresulting graphene sheets were then exposed to Oleum (—SO₃) vapor foroxidation for 1 hour to produce graphene oxide sheets.

The invention claimed is:
 1. A method of producing isolated grapheneoxide sheets directly from a graphitic material, said method comprising:a) mixing multiple particles of a graphitic material, and multipleparticles of a solid carrier material to form a mixture in an impactingchamber of an energy impacting apparatus; b) operating said energyimpacting apparatus with a frequency and an intensity for a length oftime sufficient for peeling off graphene sheets from said graphiticmaterial and transferring said graphene sheets to surfaces of said solidcarrier material particles to produce graphene-coated solid carrierparticles inside said impacting chamber; and c) sequentially orconcurrently oxidizing and separating said graphene sheets from saidsurfaces of said solid carrier material particles to produce saidisolated graphene oxide sheets, wherein said step (c) comprises exposingsaid graphene sheets to an oxidizing medium, before, during or after thegraphene sheets are separated from said solid carrier material particlesurfaces, wherein said oxidizing medium is selected from an oxidizinggas or vapor, an oxidizing plasma, or an oxidizing liquid, wherein saidoxidizing liquid is selected from a liquid comprising an oxidizerselected from sodium permanganate, transition metal permanganate;wherein said step (a) includes adding an oxidizing liquid in saidmixture so that said oxidizing liquid acts to partially oxidize saidgraphene sheets during step (b).
 2. The method of claim 1, wherein aplurality of impacting balls or media are added to the impacting chamberof said energy impacting apparatus.
 3. The method of claim 1, whereinsaid solid carrier material is selected from solid particles of anorganic, polymeric, metal, glass, ceramic, or inorganic material.
 4. Themethod of claim 1, wherein said solid carrier material is selected fromthe group consisting of plastic beads, plastic pellets, wax pellets,polymer powder or polymer reactor spheres, glass beads or fibers, metalparticles or wires, metal oxide particles, ceramic particles, andcombinations thereof.
 5. The method of claim 1, wherein said solidcarrier material includes micron- or nanometer-scaled particles that canbe dissolved in a solvent, melted above a melting temperature, etchedaway using an etching agent, vaporized or sublimed away, or burned off,and said step (c) includes a procedure of dissolving, melting, etching,vaporizing, subliming, or burning off said solid carrier material forseparating said graphene sheets.
 6. The method of claim 1, wherein saidstep (c) includes a procedure of dissolving, melting, etching,vaporizing, subliming, burning off, or ultrasonicating said solidcarrier material for separating said graphene sheets.
 7. The method ofclaim 1, wherein said step (c) comprises oxidizing said graphene sheetson said solid carrier material particle surfaces in an oxidizing liquidmedium while being submitted to ultrasonication.
 8. The method of claim1 wherein said graphitic material is selected from the group consistingof natural graphite, synthetic graphite, highly oriented pyrolyticgraphite, graphite fiber, graphitic nanofiber, graphite fluoride,oxidized graphite, chemically modified graphite, exfoliated graphite,recompressed exfoliated graphite, expanded graphite, mesocarbonmicrobead, and combinations thereof.
 9. The method of claim 1, whereinthe energy impacting apparatus is a vibratory ball mill, planetary ballmill, high energy mill, basket mill, agitator ball mill, continuous ballmill, stirred ball mill, pressurized ball mill, freezer mill, vibratorysieve, ultrasonic homogenizer mill, or resonant acoustic mixer.
 10. Themethod of claim 1, wherein said graphitic material contains anon-intercalated and non-oxidized graphitic material that has never beenpreviously exposed to a chemical or oxidation treatment prior to saidmixing step.
 11. The method of claim 1 wherein said graphene oxidesheets contain single-layer graphene oxide sheets.
 12. The method ofclaim 1 wherein said graphene oxide sheets contain at least 80%single-layer graphene oxide or at least 80% few-layer graphene oxidehaving no greater than 10 graphene planes.
 13. The method of claim 1,further comprising a step of exposing said graphene oxide to afunctionalizing agent to produce functionalized graphene oxide.
 14. Themethod of claim 13 wherein said functionalizing agent contains achemical functional group selected from the group consisting of alkyl oraryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group,amine group, sulfonate group (—SO₃H), aldehydic group, quinoidal,fluorocarbon, and combinations thereof.
 15. The method of claim 13,wherein said functionalizing agent contains an azide compound selectedfrom the group consisting of 2-azidoethanol, 3-azidopropan-1-amine,4-(2-azidoethoxy)-4-oxobutanoic acid,2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R-)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.
 16. The method of claim 13, wherein saidfunctionalizing agent contains an oxygenated group selected from thegroup consisting of hydroxyl, peroxide, ether, keto, and aldehyde. 17.The method of claim 13, wherein said functionalizing agent contains afunctional group selected from the group consisting of SO₃H, COOH, NH₂,OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃,Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ andMg—X; wherein y is an integer equal to or less than 3, R′ is hydrogen,alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl orcycloaryl, X is halide, and Z is carboxylate or trifluoroacetate, andcombinations thereof.
 18. The method of claim 13, wherein saidfunctionalizing agent contains a functional group selected from thegroup consisting of amidoamines, polyamides, aliphatic amines, modifiedaliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides,ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA),tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxyadduct, phenolic hardener, non-brominated curing agent, non-aminecuratives, and combinations thereof.
 19. The method of claim 13, whereinsaid functionalizing agent contains a functional group selected from OY,NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is afunctional group of a protein, a peptide, an amino acid, an enzyme, anantibody, a nucleotide, an oligonucleotide, an antigen, or an enzymesubstrate, enzyme inhibitor or the transition state analog of an enzymesubstrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X,R′N⁺(R′)₃X⁻, R′SiR'₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′,R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O)_(w)—R′,(C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than200.
 20. The method of claim 1, wherein said procedure of operating saidenergy impacting apparatus is conducted in a continuous manner using acontinuous energy impacting device.